Fix subset dropdown

[jerboa88]

Fixes for the subset dropdown as it doesn't work currently:

• Changed event handler as it was calling the wrong one and doing nothing
• Updated event handler to work with search functionality
• Created a local storage key for the subset value as previously it was undefined

I also added an all entry to the dropdown so that users can view fonts from all subsets, but I left that change out of this PR because I'm not sure if that's something you want to include. If so, I can add a commit for it as it's only a couple of lines.

[jerboa88]

Ah yes, I missed the call to loadVisibleFonts(). I removed the checks for the khmer subset and haven't seen any issues so I'm not sure what that was for either.

I added debouncing to the dropdown and searchbox because calling loadVisibleFonts() in the filterFonts() function made it 3-4x slower. It still ran okay on my machine but I'm sure that could have caused issues for ppl (I tried using throttle() for this but it calls the function immediately which isn't great when you're trying to type text).